Current-carrying lines are often fabricated from materials such as copper and its alloys, which are readily corrosible in different atmospheres. In particular, packages of microelectronic circuitry commonly comprise a substrate, at least one layer of copper current-carrying lines, at least one layer of an organic dielectric material (such as polyimide) which serves as an insulator, and additional layers of current-carrying lines. These structures have to be heat-treated in order to cure the organic dielectric.
During the curing cycles these organic materials often release water as a by-product. Also, even after these organic dielectrics are cured, they remain permeable to water and to other atmospheric contaminants. If the package incorporates copper or some other oxidizable or corrosible metal or alloy, the vulnerable metal must be protected in order to prevent the degradation of the circuitry over time.
Although many possible protective coating materials can be envisioned, the potential number of suitable materials is severely limited by another requirement of the electronic package. It is often the situation that there is physical contact established between one metal or alloy and another. During the package processing steps, such as curing, soldering, and brazing, the structure is often exposed to thermal cycling for prolonged periods of time. Unless the regions of metal-to-metal contact are prevented from interdiffusion during the periods of thermal cycling, the electronic properties of the package will be altered. In particular, interdiffusion between metals will adversely affect the conductivity of the metals. For current-carrying lines of small dimensions, this is a particularly severe problem.
Thus, the current-carrying lines have to be protected against all sorts of chemical attack, including oxidation, and must be protected against interdiffusion with other metals which the current-carrying lines contact. Chemical attack and interdiffusion will not only impair the conductivity of the current-carrying lines, but may also adversely affect the adhesion of these lines to an organic dielectric. For example, it is known in the art that polyimides do not adhere well to copper, and consequently an adhesion layer (such as Cr) is used between the copper and the polyimide. The poor adhesion of copper to polyimide may be due to the formation of a loosely adhering copper oxide on the surface of the copper.
These problems are generally known in the art, and reference is made to the following examples of background art:
J. E. Turn and E. L. Owen, "Metallic Diffusion Barriers for the Copper-Electrodeposited Gold System", AES Research Project, Plating, Nov. 1974, at page 1915. PA0 M. R. Pinnel et al, "Mass Diffusion in Polycrystalline Copper/Electroplated Gold Planar Couples", Metallurgical Transactions, Vol. 3, July 1972, p. 1989. PA0 R. W. Lindsay et al, "The Structure and Mechanical Properties of Electroless Nickel", J. Electrochemical Soc., Vol. 112, No. 4, p. 401, Apr., 1965. PA0 U.S. Pat. Nos. 4,065,588 and 4,188,438.
Flexibility of fabrication is achieved if the coating that provides protection against chemical attack, protection against interdiffusion, and adhesion of the line is capable of electroless deposition. If the protective coating must be electroplated, a plating base would be required which would also require an additional masking step. In addition, protecting a line by plating through a mask would only cover the top of the line; electroless deposition protects all sides. After electroplating, a step to remove the contact layer would be required. Since the current-carrying lines are often extremely small, it is difficult to provide electrical contact to them for electroplating. Even if electrical lands are provided, the IR drop is great with extremely long and small lines, so the amount of current available for plating would be restricted and poor plating would often result. Thus, use of a material capable of electroless plating deposition provides a simpler method of protecting all of the exposed sides of the corrosible line. The use of techniques such as evaporation and sputtering for the deposition of the coating layer is not sufficient, since all three exposed sides of the corrosible material will not be protected.
In the prior art, materials such as Cr, Ni, and NiP have been used as protective coatings. However, these materials are not totally suitable with respect to the aforementioned interdiffusion problem, and some (such as Cr) cannot be electrolessly plated.
When Cr is electroplated, the plating solution contains catalysts which facilitate electron transfer to the Cr ions, but the current efficiency remains very low. Thus, the use of Cr as protection for all sides of a Cu line would entail the use of a complicated electrodeposition process. Although there are obscure literature references to electroless deposition of Cr, electrochemical studies of this system indicate that electroless Cr deposition cannot be achieved. In any event, it is significantly more desirable to use straightforward processing to cover all exposed sides of the current-carrying lines.
Nickel was found not to be an adequate cladding material because of its extensive interdiffusion with Cu. Electrolessly deposited NiP and NiB alloys are superior to pure Ni, but still cause a substantial increase in the resistance of copper lines. The extent of interdiffusion of these alloys with copper does not show a straightforward dependence on the concentration of the alloying atom. Cobalt diffuses with Cu to a lesser extent than does Ni, but still does not provide truly effective protection against chemical attack and a sufficient interdiffusion barrier.
Accordingly, it is an object of this invention to provide a material that offers good protection against chemical attack of underlying copper or other corrosible lines and that prevents interdiffusion between the copper or other corrosible material and any metal or alloy with which it may be in contact.
It is another object of this invention to provide an improved coating material that can be used to prevent chemical attack of corrosible lines and interdiffusion between these lines and other contacting metals.
It is another object of this invention to provide an improved protective coating material for use with copper lines, which will protect the copper lines from all types of chemical attack.
It is another object of this invention to provide a material that will protect copper lines from corrosive chemical attack, and that can be used as an interdiffusion barrier between the copper lines and other metals.
It is a further object of the present invention to protect copper lines using a coating layer that can be electrolessly deposited.
It is another object of this invention to provide an improved coating material for copper lines, which is effective when electrolessly deposited in thin layers.
It is another object of this invention to provide an improved electronic package that contains organic dielectric material and current carrying metallic lines, where the metallic lines are protected against corrosion and interdiffusion by an improved layer of protective material.
It is a further object of this invention to provide an improved protective coating for copper and other corrosible materials, which coating can be deposited as a thin film and which will prevent chemical attack of the copper or corrosible material and also interdiffusion between the copper or other corrosible material and other metals.
It is a further object of this invention to provide a protective coating layer of CoP on Cu lines.
It is another object of the present invention to provide an improved interdiffusion barrier that can be used between copper and other metals, such as gold, and that will prevent interdiffusion between the copper and these other metals.